1. Field of the Invention
The present invention relates to a liquid crystal display device having a touch panel.
2. Description of the Prior Art
Two main types of known image display devices for displaying information are CRTs (cathode-ray tubes) and liquid crystal display devices. CRTs have the disadvantages of being large and heavy and consuming much electric power. By contrast, liquid crystal display devices have the advantages of occupying minimal space, being light-weight, and consuming minimal electric power. For these reasons, liquid crystal display devices are nowadays widely used in various applications. Liquid crystal display devices are effective as display devices for use in personal computers and household appliances, and are particularly so in cellular telephones and portable digital assistants where portability is an important factor.
Liquid crystal display devices sometimes suffer from unwanted reflection, in which, for example, the face of the person viewing the liquid crystal screen or external light such as the light from the sun or a fluorescent lamp is reflected from the liquid crystal panel. This is particularly troublesome when a portable digital assistant is used outdoors, because such unwanted reflection degrades the quality of the displayed image. Moreover, under such conditions, glare (moirxc3xa9 image interference fringes) is also likely.
To suppress glare and obtain satisfactory image quality, an antiglare film is sometimes bonded on the viewer side of the polarizer plate of a liquid crystal display device. This antiglare film serves to suppress reflection of external light as mentioned above by being formed out of a transparent resin film or the like having its surface roughened. For example, such an antiglare film is produced by forming a transparent base out of a resin such as polyethylene terephthalate and then roughening its surface by etching or the like. The characteristics of an antiglare film are represented by the center-line mean roughness Ra (xcexcm) defined in JIS (Japanese Industrial Standards) B0601, the irregularity interval Sm (xcexcm), and the haze value H (%) defined in JIS K7105.
Antiglare films are disclosed, for example, in Japanese Patent Applications Laid-Open Nos. H9-127312, H9-193332, and H9-193333. The antiglare film disclosed in Japanese Patent Application Laid-Open No. H9-127312 is formed such that Ra is in the range from 0.03 to 0.3 xcexcm, Sm is 50 xcexcm or less, and H is in the range from 1 to 25%.
The antiglare film disclosed in Japanese Patent Application Laid-Open No. H9-193332 is formed such that Ra is in the range from 0.1 to 0.3 xcexcm, Sm is in the range from 30 to 100 xcexcm, and H is in the range from 14 to 22%. The antiglare film disclosed in Japanese Patent Application Laid-Open No. H9-193333 is formed such that Ra is in the range from 0.05 to 0.2 xcexcm, Sm is in the range from 60 to 180 xcexcm, and H is in the range from 3 to 8%.
All of these antiglare films are to be placed at the forward end of a liquid crystal display device so as to effectively reduce unwanted reflection of external light and glare.
In the field of portable digital assistants, the market has recently been growing for portable digital assistants that are provided with a liquid crystal display device having a touch panel that allows pen-based data entry. Conventionally, the mainstream of such touch panels is the type that is bonded at the forward end of a liquid crystal display device. Among different types of touch panels, such as optical, supersonic, electromagnetic-induction-based, and resistive-film-based touch panels, resistive-film-based touch panels are typically used in portable digital assistants.
When an antiglare film is incorporated in a portable digital assistant provided with a liquid crystal display device having a touch panel, its display portion is usually assembled by arranging, from the viewer side, the touch panel, the antiglare film, a first polarizer plate, a first phase-difference plate, a liquid crystal display element, a second phase-difference plate, and a second polarizer plate. As is well known, here, the polarizer plates are used to extract light polarized in a particular direction, and the phase-difference plates are used to compensate colors optically. As the liquid crystal display element is used, for example, an STN (super-twisted nematic) type liquid crystal display element.
As shown in FIG. 3, a resistive-film-based touch panel 3 is formed by arranging, from the viewer side, a film 11, an electrode 12a, a spacer 13, an electrode 12b, and a glass plate 14. This touch panel 3 as a whole is bonded, with double-faced adhesive tape 15, to an antiglare film 2 that is placed on top of a liquid crystal display element 1.
However, a resistive-film-based touch panel 3 as shown in FIG. 3 functions as a data entry switch, with the electrodes 12a and 12b, each formed out of a transparent conductive film, serving as conductors. Accordingly, if this touch panel 3 is used together with an antiglare film 2 of which the characteristics Ra, Sm, and H are defined as disclosed in Japanese Patent Applications Laid-Open Nos. H9-127312, H9-193332, and H9-193333 mentioned above, the light reflected from the forward-end surface of the touch panel 3 and from the individual layers constituting it (i.e. the electrodes 12a and 12b, the glass plate 15, etc.) interferes with the light reflected from the surface of the antiglare film 2 placed on top of the liquid crystal display element 1.
This rather aggravates glare in the displayed image, and thereby greatly degrades viewability. On the other hand, if the antiglare film 2 is removed, glare is suppressed, but unwanted reflection degrades the quality of the displayed image.
An object of the present invention is to provide a liquid crystal display device having a touch panel that offers satisfactory image quality with a minimum of unwanted reflection and glare.
To achieve the above object, according to one aspect of the present invention, in a liquid crystal display device having a touch panel that exploits a shutter effect of a liquid crystal layer, an antiglare film is placed behind the touch panel as seen from the viewer and the antiglare film fulfills conditions (1), (2), and (3) below:
0.3xe2x89xa6Raxe2x89xa60.4xe2x80x83xe2x80x83(1)
50xe2x89xa6Smxe2x89xa680xe2x80x83xe2x80x83(2)
5xe2x89xa6Hxe2x89xa620xe2x80x83xe2x80x83(3)
where Ra represents the center-line mean roughness (xcexcm) of the surface of the antiglare film, Sm represents the irregularity interval (xcexcm) of the surface, and H represents the haze value (%).
According to another aspect of the present invention, in a liquid crystal display device having a touch panel that exploits a shutter effect of a liquid crystal layer, the touch panel, an antiglare film, a first polarizer plate, a first phase-difference plate, an STN-type liquid crystal display element, a second phase-difference plate, and a second polarizer plate are arranged in this order from the viewer side, and the antiglare film fulfills conditions (1), (2), and (3) above.
In this liquid crystal display device, the touch panel may be of a type that employs a resistive film, and in addition a gap may be secured between the touch panel and the antiglare film.